The present invention relates to improvements in LSI circuits and, more particularly, in high-speed and low-power LSI circuits composed of extremely small elements.
With the widespread use of portable equipment in recent years, LSIs consuming lower power have been in greater demand. To implement a lower-power LSI, a power-source voltage used in the LSI has been lowered steadily. However, the lowering of the internal power-source voltage entails a rapid reduction in the operating speed of the circuit. To effectively overcome the problem, the threshold voltage of a transistor may be lowered (Low-Vt). When the threshold voltage of the transistor is lowered, a current flowing through the transistor is increased sufficiently to increase the operating speed thereof. However, a leakage current flowing through the transistor in the OFF state (OFF-state leakage current) is also increased disadvantageously in the standby or active state. Moreover, if a variation in manufacturing process for the transistor causes a variation in the threshold voltage thereof, the variation may greatly affect the lowered threshold voltage.
To overcome the problem of the OFF-state leakage current increased in the standby state, there has been proposed a technique disclosed in Japanese Laid-Open Patent Publication HEI 6-208790 whereby a potential at the source node of a transistor being cut off in the standby state is varied in such a direction as to decrease the leakage current so that the leakage current flowing through the circuit in the standby state is reduced.
However, since the conventional technique has varied the potential at the source node of the transistor on the assumption that the threshold voltage of the transistor is constant, serious problems occur if a variation in manufacturing process therefor or a temperature variation during the use of a finished product cause a variation in the threshold voltage of the transistor. Specifically, an increase in the threshold voltage of the transistor decreases the operating speed of the circuit, while a decrease in the threshold voltage of the transistor increases the leakage current flowing through the transistor in the OFF state.